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This study explores the capability of amphibole in tracing the physicochemical process of magma mixing through spatially associated gabbros, mafic microgranular enclaves (MMEs) and granodiorites from central Tibet. These rocks share similar zircon ages as well as zircon Hf‐O and plagioclase Sr isotopes. However, the amphiboles within the gabbros and granodiorites have different Sr and B isotope compositions, while amphiboles with both heterogeneous isotopic imprints occur in the MMEs. According to data and modeling, significant mixing of two isotopically distinct magmas is recorded by amphibole but not by zircon and plagioclase. Based on a synthesis of petrography, geochemistry and thermobarometry, we interpret this inconsistency by the crystallization order of minerals and propose that magma mixing occurred after the parent magma was emplaced at ∼10 km and cooled to ∼750°C. Our study highlights that amphibole may be a more sensitive tracer of magma mixing relative to other commonly used methods. Plain Language Summary: Magma mixing significantly shapes the composition of silicic igneous rocks representing the major constituent of the upper continental crust. Since bulk‐rock composition may only bear the average of mixed sources, in situ techniques such as Hf‐O isotopes in zircon and Sr‐Pb isotopes in plagioclase are widely used to investigate the details of magma mixing. However, these methods cannot constrain how magma mixing operates in the deep crust. This study novelly uses in situ Sr‐B isotopes and trace elements in amphibole to trace the physicochemical process of magma mixing through spatially associated gabbros, mafic microgranular enclaves and granodiorites from the central Tibetan Plateau. Our data show that significant mixing of two isotopically distinct magmas is recorded by amphibole but not by zircon and plagioclase. We demonstrate that the amphibole geochemistry (i.e., trace element, Sr‐B isotope compositions) may be more sensitive in tracing magma mixing relative to traditional isotopic tools and has the potential to unravel the physicochemical process(es) of magma mixing in the deep crust. In addition, our work reinforces the use of B isotopes in amphibole to discern the nature (fluids released from altered oceanic crust vs. residual slab) of the slab components that metasomatized the supra‐subduction mantle. Key Points: The amphiboles in central Tibetan magmatic complex have contrasting geochemical and Sr‐B isotopic compositionsSignificant magma mixing is well recorded by amphibole but not by zircon and plagioclaseAmphibole is a more sensitive tracer of magma mixing relative to zircon and plagioclase [ABSTRACT FROM AUTHOR]

As the largest petroliferous basin in northeast (NE) China, Songliao Basin preserves continuous Cretaceous sedimentary‐volcanic records, providing an excellent opportunity to recover the palaeo‐environment. The Yingcheng Formation in the Songliao Basin contains ample gas reservoirs that attract widespread attention. Whereas the timing, petrogenesis and geodynamic mechanism of the volcanic rocks in the Yingcheng Formation are still controversial, which largely constrain our understanding of the formation and evolution of the Songliao Basin. Here, we present an integrated investigation of zircon U–Pb ages and Hf isotope, as well as whole‐rock elements and Sr‐Nd isotopes data for a suite of rhyolites of Yingcheng Formation from the SK2 Borehole of NE China. Zircon U–Pb age dating results of nine samples have yielded a concentrated age of ~110 Ma. These Yingcheng rhyolites are characterized by high SiO2 (66–78 wt%) and alkali (Na2O + K2O = 7.80–11.70 wt%) content, high 10000*Ga/Al (1.26–3.82, mostly >2.6) and FeOT/(FeOT + MgO) ratios (0.81–0.95), which show geochemical affinities with A‐type rhyolites. They have relatively low Y/Nb (0.69–1.78, average 1.19) and Rb/Nb (1.58–6.52, average 4.34) ratios, suggesting that the Yingcheng volcanic samples belong to A1‐type rhyolites which formed in an intraplate environment. These Yingcheng rhyolites show depleted Nd‐Hf isotopic compositions (εNd(t) = 2.43–4.87 and zircon εHf(t) = 4.22–9.88) comparable with the Early Cretaceous A‐type and I‐type rhyolites in the Songliao Basin, suggesting that they were originated from a juvenile continental crust. They were most likely derived from the partial melting of anhydrous lower crust instead of differentiation of mantle‐derived or mixing with alkaline basaltic magma, due to their low Mg# and absence of the coeval mafic rocks. Combined with the previous studies, A‐type rhyolites in the Yingcheng Formation erupted lasting at least 10 Myr and were widely distributed in the rifts of the Songliao Basin. We propose that the Yingcheng rhyolites were most likely generated from the rollback of the Palaeo‐Pacific Plate in the Early Cretaceous, thus highlighting the significance of the subduction of the Palaeo‐Pacific Plate during the secular evolution of the Songliao Basin. [ABSTRACT FROM AUTHOR]

The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.705006–0.705963, εNd(t) = −1.78 to 3.52, zircon εHf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO2, Al2O3, Sr/Y, 87Sr/86Sr, εNd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts. [ABSTRACT FROM AUTHOR]

Studying granitoids can provide important knowledge on the composition of continental crust and the geodynamic processes of continental growth. Nevertheless, the mechanisms of granitoids formation during continental collision remain uncertain. In this paper, we present new zircon U–Pb–Hf isotope and whole‐rock Sr–Nd isotope compositions, and major‐ and trace‐element data for granitoids from the Chacangka area of the central Lhasa Terrane, southern Tibet. The Chacangka quartz monzonites crystallized at ca. 61 Ma, whereas the monzonites and granites were emplaced at ca. 51 Ma. The quartz monzonites are peraluminous and have high 87Sr/86Sri ratios (0.7129–0.7131), and their low εNd(t) (−7.65 to −7.10) and zircon εHf(t) (−9.3 to −7.5) values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane, which suggests that they originated from a mixed source of juvenile and ancient lower crust. Compared with the quartz monzonites, the monzonites have much higher contents of MgO (2.34–3.62 wt%) and values of Mg# (42.3–46.3) and zircon εHf(t) (−6.9 to 1.9) and show more depleted Sr–Nd isotopes, indicating that more juvenile materials were involved in their source. The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher SiO2 contents, lower CaO and MgO contents, and lower values of Mg#, suggesting that they were differentiates of the monzonites. The elemental and isotopic compositions of the Chacangka granitoids exhibit a marked transformation from 61 to 51 Ma, probably indicating breakoff of the Neo‐Tethyan oceanic slab. The Neo‐Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re‐melting and destruction of ancient crustal basement of the central Lhasa Terrane. [ABSTRACT FROM AUTHOR]

Southern Tibet is a key region for investigating the magmatism that occurredduring the collision between India and Asia. The break‐off of the Neo‐Tethyan slab, after the onset of continental collision at ca. 65–60 Ma, affected the lithospheric thermal regime and led to a series of magmatic and tectonic events. Many magmatic rocks generated by slab break‐off have been reported in southern Tibet; however, which of these record the initial slab break‐off (i.e., slab tearing) is still an open question. The Lianxiang gabbros, which intrude an ophiolite in central‐southern Tibet, provide an ideal opportunity to investigate the footprint of the initial slab break‐off. Zircon U–Pb ages show that the Lianxiang gabbros crystallized at 57–53 Ma, coeval with other mafic dikes in southern Tibet. The Lianxiang gabbros are characterized by high Na2O (2.13–5.42 wt%), Al2O3 (13.86–17.13 wt%), and V (188–346 ppm) contents and moderate Mg# (40.8–65.9). The gabbrosare slightly depleted in light rare earth elements [LREE; (La/Yb)N = 0.45–0.76], similar to normal mid‐ocean ridge basalt (N‐MORB). They also have low 87Sr/86Sri ratios (0.703538–0.705698) and high εNd(t) values (9.49–10.68), indicating that they were derived mainly from a shallow asthenospheric mantle source. Combining our results with data from other magmatism at ~57 Ma in the southern Lhasa Terrane, we propose that the N‐MORB‐type rocks reflect the tearing of the Neo‐Tethyan slab. The process of slab break‐off differed between western and eastern southern Tibet, possibly as a result of different lithospheric structures and compositions. [ABSTRACT FROM AUTHOR]